Optical communication between face-to-face semiconductor chips

ABSTRACT

One embodiment of the present invention provides a system that communicates between a first semiconductor die and a second semiconductor die through optical signaling. During operation, the system converts an electrical signal into an optical signal using an electrical-to-optical transducer located on a face of the first semiconductor die, wherein the first semiconductor die and the second semiconductor die are oriented face-to-face so that the optical signal generated on the first semiconductor die shines on the second semiconductor die. Upon receiving the optical signal on a face of the second semiconductor die, the system converts the optical signal into a corresponding electrical signal using an optical-to-electrical transducer located on the face of the second semiconductor die.

RELATED APPLICATION

[0001] This application hereby claims priority under 35 U.S.C. 119 toU.S. Provisional Patent Application No. 60/460,104, filed on 3 Apr.2003, entitled, “Optical Communication for Face to Face Chips,” byinventors Robert J. Drost and William C. Coates (Attorney Docket No.SUN-P9705PSP).

GOVERNMENT LICENSE RIGHTS

[0002] This invention was made with United States Government supportunder Contract No. NBCH020055 awarded by the Defense Advanced ResearchProjects Administration. The United States Government has certain rightsin the invention.

BACKGROUND

[0003] 1. Field of the Invention

[0004] The present invention relates to techniques for communicatingbetween integrated circuit chips. More specifically, the presentinvention relates to a method and an apparatus for communicating betweenintegrated circuit chips that are arranged face-to-face through opticalsignaling.

[0005] 2. Related Art

[0006] Advances in semiconductor technology presently make it possibleto integrate large-scale systems, including tens of millions oftransistors, into a single semiconductor chip. Integrating suchlarge-scale systems onto a single semiconductor chip increases the speedat which such systems can operate, because signals between systemcomponents do not have to cross chip boundaries, and are not subject tolengthy chip-to-chip propagation delays. Moreover, integratinglarge-scale systems onto a single semiconductor chip significantlyreduces production costs, because fewer semiconductor chips are requiredto perform a given computational task.

[0007] Unfortunately, these advances in semiconductor technology havenot been matched by corresponding advances in inter-chip communicationtechnology. Semiconductor chips are typically integrated onto a printedcircuit board that contains multiple layers of signal lines forinter-chip communication. However, signal lines on a semiconductor chipare about 100 times more densely packed than signal lines on a printedcircuit board. Consequently, only a tiny fraction of the signal lines ona semiconductor chip can be routed across the printed circuit board toother chips. This problem creates a bottleneck that continues to grow assemiconductor integration densities continue to increase.

[0008] Researchers have begun to investigate alternative techniques forcommunicating between semiconductor chips. One promising techniqueinvolves integrating arrays of capacitive transmitter plates andreceiver plates onto semiconductor chips to facilitate inter-chipcommunication. If a first chip is situated face-to-face with a secondchip so that transmitter plates on the first chip are capacitivelycoupled with receiver plates on the second chip, it becomes possible totransmit signals directly from the first chip to the second chip withouthaving to route the signal through intervening signal lines within aprinted circuit board. It is also possible to communicate in a similarmanner through inductive coupling by using wire loops to couple magneticfields between chips.

[0009] Unfortunately, both capacitive and inductive coupling mechanismsdecrease in strength by approximately the inverse of the distancebetween the chips. The decreased strength of the received signal reducesrobustness of the communication mechanism and increases the complexity,power, and area of the receiver and transmitter circuits. Additionally,capacitive and inductive coupling mechanisms suffer from cross-talk, ordestructive coupling, from adjacent channels that increases with thedistance between chips. It may consequently be difficult to assemblechips into a system with sufficient precision to allow capacitive andinductive coupling mechanisms to operate effectively.

[0010] Hence, what is needed is a method and an apparatus forcommunicating between semiconductor chips without the above-describedproblems.

SUMMARY

[0011] One embodiment of the present invention provides a system thatcommunicates between a first semiconductor die and a secondsemiconductor die through optical signaling. During operation, thesystem converts an electrical signal into an optical signal using anelectrical-to-optical transducer located on a face of the firstsemiconductor die, wherein the first semiconductor die and the secondsemiconductor die are oriented face-to-face so that the optical signalgenerated on the first semiconductor die shines on the secondsemiconductor die. Upon receiving the optical signal on a face of thesecond semiconductor die, the system converts the optical signal into acorresponding electrical signal using an optical-to-electricaltransducer located on the face of the second semiconductor die.

[0012] In a variation on this embodiment, after generating the opticalsignal on the first semiconductor die, the system passes the opticalsignal through annuli located within metal layers on the firstsemiconductor die to focus the optical signal onto the secondsemiconductor die.

[0013] In a variation on this embodiment, after generating the opticalsignal on the first semiconductor die, the system uses a lens to focusthe optical signal onto the second semiconductor die.

[0014] In a variation on this embodiment, after generating the opticalsignal on the first semiconductor die, the system uses a mirror toreflect the optical signal, so that the optical signal can shine on thesecond semiconductor die without the first semiconductor die having tobe coplanar with the second semiconductor die.

[0015] In a variation on this embodiment, after generating the opticalsignal on the first semiconductor die, the system passes the opticalsignal through an interposer sandwiched between the first semiconductordie and the second semiconductor die. This interposer contains one ormore waveguides that direct the optical signal, so that the opticalsignal shines on the second semiconductor die.

[0016] In a variation on this embodiment, the electrical-to-opticaltransducer is a member of a plurality of electrical-to-opticaltransducers located on the first semiconductor die, and theoptical-to-electrical transducer is a member of a plurality ofoptical-to-electrical transducers located on the first semiconductordie. In this variation, a plurality of optical signals can betransmitted in parallel from the first semiconductor die to the secondsemiconductor die.

[0017] In a further variation, multiple spatially adjacentelectrical-to-optical transducers in the plurality ofelectrical-to-optical transducers transmit the same signal, andelectronic steering circuits in the first semiconductor die direct datato the multiple spatially adjacent electrical-to-optical transducers tocorrect mechanical misalignment in X, Y and Θ coordinates.

[0018] In a further variation, multiple spatially adjacentoptical-to-electrical transducers in the plurality ofoptical-to-electrical transducers receive the same signal, andelectronic steering circuits in the second semiconductor die direct datafrom the multiple spatially adjacent optical-to-electrical transducersto correct mechanical misalignment in X, Y and Θ coordinates.

[0019] In a variation on this embodiment, the electrical-to-opticaltransducer can be: a Zener diode, a light emitting diode (LED), avertical cavity surface emitting laser (VCSEL), or an avalanchebreakdown P-N diode.

[0020] In a variation on this embodiment, the optical-to-opticaltransducer can be: a P-N-diode photo-detector, or a P-I-N-diodephoto-detector.

BRIEF DESCRIPTION OF THE FIGURES

[0021]FIG. 1 illustrates a checkerboard pattern for integrated circuitchips that communicate through face-to-face signaling in accordance withan embodiment of the present invention.

[0022]FIG. 2A presents a cross-sectional view of face-to-face chipsaccordance with an embodiment of the present invention.

[0023]FIG. 2B illustrates a communication region for face-to-face chipsaccordance with an embodiment of the present invention.

[0024]FIG. 3 illustrates six degrees of alignment between semiconductorchips in accordance with an embodiment of the present invention.

[0025]FIG. 4 illustrates how a communication channel can includemultiple channel components in accordance with an embodiment of thepresent invention.

[0026]FIG. 5 illustrates an on-chip metal structure that directs anoptical beam in accordance with an embodiment of the present invention.

[0027]FIG. 6 presents an offset-view of an on-chip metal structure thatdirects an optical beam in accordance with an embodiment of the presentinvention.

[0028]FIG. 7A illustrates line-of-sight optical paths in accordance withan embodiment of the present invention.

[0029]FIG. 7B illustrates focused optical paths in accordance with anembodiment of the present invention.

[0030]FIG. 7C illustrates reflected optical paths in accordance with anembodiment of the present invention.

[0031]FIG. 8 illustrates an interposer containing an array of waveguides in accordance with an embodiment of the present invention.

[0032]FIG. 9 presents a flow chart illustrating the process ofcommunicating between chips through optical signaling in accordance withan embodiment of the present invention.

DETAILED DESCRIPTION

[0033] The following description is presented to enable any personskilled in the art to make and use the invention, and is provided in thecontext of a particular application and its requirements. Variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to other embodiments and applications without departing fromthe spirit and scope of the present invention. Thus, the presentinvention is not limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

[0034] Arrangement for Chip-to-Chip Communication

[0035] In FIG. 1, chips 101-113 are arranged to communicate with eachother through face-to-face overlapping regions in their corners. In thisarrangement, each chip can communicate with four neighboring chips. Notethat many other arrangements that facilitate face-to-face communicationwill be obvious to a practitioner with ordinary skill in the art.

[0036] In one embodiment of the present invention, the arrangement ofchips illustrated in FIG. 1 comprises a computer system, where at leastone chip, such as chip 104, contains one or more processors, and whereinneighboring chips, 101, 102, 106 and 107, contain circuitry thatcommunicates with the one or more processors in chip 104.

[0037]FIGS. 2A and 2B illustrate two chips 101 and 104 that communicatewith each other through proximity communication. As is illustrated inFIG. 2A, chips 101 and 104 are positioned so that they overlap with theactive face of chip 101 facing the active face of chip 104. (Note thatthe term “active face” refers to the face of the integrated circuit thatcontains active circuitry.) The region of overlap is referred to as the“communication region 202” through which transmitter and receivercircuits communicate using optical signals.

[0038] As shown in FIG. 2B, communication region 202 comprises a numberof communication channels, including channel 204. One motivation forproximity communication is to use modern fine-line chip lithographyfeatures to pack transmitter and receiver channels close to one another.Note that off-chip bonding and wiring methods such as wire-bonds, ballgrid arrays, and circuit board traces have pitches on the order of 100microns, whereas on-chip wiring can have pitches of less than micron.Hence, we can pack proximity communication channels on a very tightpitch, for example on the order of 5-30 microns. Packing channels on atight pitch enables high-bandwidth communication between chips, but alsocreates challenges in achieving the requisite alignment tolerances.

[0039]FIG. 3 illustrates six coordinates of alignment between the planarsurfaces of chip A and chip B. Misalignments in the X, Y, and Θcoordinates cause shifts and rotations between the chips surfaces.

[0040] It is possible to correct for mechanical misalignments (shifts orrotations) between the chip's surfaces by subdividing the transmitter,receiver, or both transmitter and receiver for each channel as shown inFIG. 4 into an array of channel components which we “call micro-pads.”Electronic steering circuits can then be used to direct data to and fromthe micro-pads to correct mechanical misalignments in X, Y and Θcoordinates.

[0041] However, this technique cannot correct for mechanicalmisalignment in Z, Ψ, and Φ coordinates. Misalignments in the Z, Ψ, andΦ coordinates cause gaps between the chips that reduced signal strengthas the chips become distant. Moreover, cross-talk significantlyincreases as the chip separation increases.

[0042] One embodiment of the present invention uses optical signalingtechniques for proximity communication. In contrast to capacitive orinductive coupling techniques, optical signaling techniques can usefocused or coherent light that falls off more slowly with Z-distance andcauses less crosstalk. Hence, optical signaling can alleviate thesensitivity to gaps, or Z-distance, between the chips.

[0043] As illustrated in FIG. 4, a channel 204 may include one or morechannel components, such as channel component 402. More specifically,channel 204 may include one or more electrical-to-optical transducerelements for the transmitter end of the channel, and may include one ormore optical-to-electrical transducer elements for the receiver end ofthe channel. Example electrical-to-optical transducer elements includelight-emitting diodes and vertical-cavity surface-emission lasers(VCSELs). Example optical-to-electrical transducer elements include P-Ndiodes or P-I-N diodes. Standard CMOS fabrication technologies can beused to create light-emitting diodes and P-N diodes. However, VCSELs andP-I-N diodes require fabrication with 3-5 materials such asgallium-arsenide, or special fabrication steps in a CMOS process,respectively.

[0044] In order to increase the signal-to-noise ratio it is desirable todirect the light from the transmitter end to the receiver end of thecircuit so each channel's optical energy stays mostly within thechannel. We refer to this directing process as “focusing” the light.

[0045]FIG. 5 illustrates an on-chip mechanism that focuses light. InFIG. 5, an annular ring 502 surrounds a light source 504. Optical energyis directed from light source 504 through a constrained path in theopening of annular ring 502, which reduces transverse spreading of theoptical energy. The annular ring structure can be repeated on multiplechip metallization layers to concentrate the beam further as illustratedin FIG. 6.

[0046]FIGS. 7A, 7B and 7C illustrate three off-chip mechanisms fordirecting light from the transmitter end to the receiver end of thechannels. FIG. 7A illustrates line-of-sight optical paths, in whichchips are simply aligned so that the transmitted beams of light fall onthe receiving structures. FIG. 7B illustrates focused optical paths, inwhich a lens structure 702 is used to focus the transmitted light. Thisfocused optical path mechanism is more complex, but compensates forspreading of the beam of light. FIG. 7C illustrates a reflected opticalpath mechanism, which uses a reflector, such as mirror 704, to permitthe transmitter end and receiver ends of the channel to be non-coplanar.Note that the reflected path mechanism can be combined with the focusedpath method.

[0047] Finally, FIG. 8 illustrates another off-chip mechanism that usesan interposer 802, which contains an array of embedded waveguides todirect light from the transmitter end to the receiver end of thechannel. A typical optical waveguide includes an optically transparentmaterial with two or more indices of refractivity. The channel in awaveguide has a higher index of refraction that the surrounding claddingmaterial, so that light in the channel reflects off the walls of thewaveguide and remains contained in the channel.

[0048]FIG. 9 presents a flow chart illustrating the process ofcommunicating between chips through optical signaling in accordance withan embodiment of the present invention. The process starts when anelectrical signal is converted into an optical signal through anelectrical-to-optical transducer located on a first semiconductor die(step 902). Next, the optical signal is directed onto a secondsemiconductor die using any of the techniques described above withrespect to FIG. 5, FIG. 6, FIGS. 7A-7C and FIG. 8 (step 904). Finally,the optical signal is converted in to a corresponding electrical signalthrough an optical-to-electrical transducer located on the secondsemiconductor die (step 906).

[0049] The foregoing descriptions of embodiments of the presentinvention have been presented only for purposes of illustration anddescription. They are not intended to be exhaustive or to limit thepresent invention to the forms disclosed. Accordingly, manymodifications and variations will be apparent to practitioners skilledin the art. Additionally, the above disclosure is not intended to limitthe present invention. The scope of the present invention is defined bythe appended claims.

What is claimed is:
 1. A method for communicating between a firstsemiconductor die and a second semiconductor die through opticalsignaling, comprising: converting an electrical signal into an opticalsignal using an electrical-to-optical transducer located on a face ofthe first semiconductor die; wherein the first semiconductor die and thesecond semiconductor die are oriented face-to-face so that the opticalsignal generated on the first semiconductor die shines on the secondsemiconductor die; receiving the optical signal on a face of the secondsemiconductor die; and converting the optical signal into acorresponding electrical signal using an optical-to-electricaltransducer located on the face of the second semiconductor die.
 2. Themethod of claim 1, wherein after generating the optical signal on thefirst semiconductor die, the method further comprises passing theoptical signal through annuli located within metal layers on the firstsemiconductor die to focus the optical signal onto the secondsemiconductor die.
 3. The method of claim 1, wherein after generatingthe optical signal on the first semiconductor die, the method furthercomprises using a lens to focus the optical signal onto the secondsemiconductor die.
 4. The method of claim 1, wherein after generatingthe optical signal on the first semiconductor die, the method furthercomprises using a mirror to reflect the optical signal, so that theoptical signal can shine on the second semiconductor die without thefirst semiconductor die having to be coplanar with the secondsemiconductor die.
 5. The method of claim 1, wherein after generatingthe optical signal on the first semiconductor die, the method furthercomprises passing the optical signal through an interposer sandwichedbetween the first semiconductor die and the second semiconductor die,wherein the interposer contains one or more waveguides that direct theoptical signal, so that the optical signal shines on the secondsemiconductor die.
 6. The method of claim 1, wherein theelectrical-to-optical transducer is a member of a plurality ofelectrical-to-optical transducers located on the first semiconductordie; and wherein the optical-to-electrical transducer is a member of aplurality of optical-to-electrical transducers located on the firstsemiconductor die; whereby a plurality of optical signals can betransmitted in parallel from the first semiconductor die to the secondsemiconductor die.
 7. The method of claim 6, wherein multiple spatiallyadjacent electrical-to-optical transducers in the plurality ofelectrical-to-optical transducers transmit the same signal; and whereinelectronic steering circuits in the first semiconductor die direct datato the multiple spatially adjacent electrical-to-optical transducers tocorrect mechanical misalignment in X, Y and Θ coordinates.
 8. The methodof claim 6, wherein multiple spatially adjacent optical-to-electricaltransducers in the plurality of optical-to-electrical transducersreceive the same signal; and wherein electronic steering circuits in thesecond semiconductor die direct data from the multiple spatiallyadjacent optical-to-electrical transducers to correct mechanicalmisalignment in X, Y and Θ coordinates.
 9. The method of claim 1,wherein the electrical-to-optical transducer includes one of: a Zenerdiode; a light emitting diode (LED); a vertical cavity surface emittinglaser (VCSEL); and an avalanche breakdown P-N diode.
 10. The method ofclaim 1, wherein the optical-to-optical transducer includes one of: aP-N-diode photo-detector; and a P-I-N-diode photo-detector.
 11. Anapparatus for communicating between semiconductor chips through opticalsignaling, comprising: a first semiconductor die; a second semiconductordie; an electrical-to-optical transducer located on a face of the firstsemiconductor die, which is configured to convert an electrical signalinto an optical signal; wherein the first semiconductor die and thesecond semiconductor die are oriented face-to-face so that the opticalsignal generated on the first semiconductor die shines on the secondsemiconductor die; an optical-to-electrical transducer located on a faceof the second semiconductor die, which is configured to convert theoptical signal received from the first semiconductor die into acorresponding electrical signal.
 12. The apparatus of claim 11, furthercomprising annuli located within metal layers on the first semiconductordie configured to focus the optical signal onto the second semiconductordie.
 13. The apparatus of claim 11, further comprising a lens configuredto focus the optical signal onto the second semiconductor die.
 14. Theapparatus of claim 11, further comprising a mirror configured to reflectthe optical signal, so that the optical signal can shine on the secondsemiconductor die without the first semiconductor die having to becoplanar with the second semiconductor die.
 15. The apparatus of claim11, further comprising an interposer sandwiched between the firstsemiconductor die and the second semiconductor die, wherein theinterposer contains one or more waveguides that direct the opticalsignal, so that the optical signal shines on the second semiconductordie.
 16. The apparatus of claim 11, wherein the electrical-to-opticaltransducer is a member of a plurality of electrical-to-opticaltransducers located on the first semiconductor die; and wherein theoptical-to-electrical transducer is a member of a plurality ofoptical-to-electrical transducers located on the first semiconductordie; whereby a plurality of optical signals can be transmitted inparallel from the first semiconductor die to the second semiconductordie.
 17. The apparatus of claim 16, wherein multiple spatially adjacentelectrical-to-optical transducers in the plurality ofelectrical-to-optical transducers transmit the same signal; and whereinelectronic steering circuits in the first semiconductor die direct datato the multiple spatially adjacent electrical-to-optical transducers tocorrect mechanical misalignment in X, Y and Θ coordinates.
 18. Theapparatus of claim 16, wherein multiple spatially adjacentoptical-to-electrical transducers in the plurality ofoptical-to-electrical transducers receive the same signal; and whereinelectronic steering circuits in the second semiconductor die direct datafrom the multiple spatially adjacent optical-to-electrical transducersto correct mechanical misalignment in X, Y and Θ coordinates.
 19. Theapparatus of claim 11, wherein the electrical-to-optical transducerincludes one of: a Zener diode; a light emitting diode (LED); a verticalcavity surface emitting laser (VCSEL); and an avalanche breakdown P-Ndiode.
 20. The apparatus of claim 11, wherein the optical-to-opticaltransducer includes one of: a P-N-diode photo-detector; and aP-I-N-diode photo-detector.
 21. A computer system includingsemiconductor chips that communicate with each other through opticalsignaling, comprising: a first semiconductor die containing one or moreprocessors; a second semiconductor die containing circuitry thatcommunicates with the one or more processors; an electrical-to-opticaltransducer located on a face of the first semiconductor die, which isconfigured to convert an electrical signal into an optical signal;wherein the first semiconductor die and the second semiconductor die areoriented face-to-face so that the optical signal generated on the firstsemiconductor die shines on the second semiconductor die; anoptical-to-electrical transducer located on a face of the secondsemiconductor die, which is configured to convert the optical signalreceived from the first semiconductor die into a correspondingelectrical signal.
 22. The computer system of claim 21, furthercomprising annuli located within metal layers on the first semiconductordie configured to focus the optical signal onto the second semiconductordie.
 23. The computer system of claim 21, further comprising a lensconfigured to focus the optical signal onto the second semiconductordie.
 24. The computer system of claim 21, further comprising a mirrorconfigured to reflect the optical signal, so that the optical signal canshine on the second semiconductor die without the first semiconductordie having to be coplanar with the second semiconductor die.
 25. Thecomputer system of claim 21, further comprising an interposer sandwichedbetween the first semiconductor die and the second semiconductor die,wherein the interposer contains one or more waveguides that direct theoptical signal, so that the optical signal shines on the secondsemiconductor die.
 26. The computer system of claim 21, wherein theelectrical-to-optical transducer is a member of a plurality ofelectrical-to-optical transducers located on the first semiconductordie; and wherein the optical-to-electrical transducer is a member of aplurality of optical-to-electrical transducers located on the firstsemiconductor die; whereby a plurality of optical signals can betransmitted in parallel from the first semiconductor die to the secondsemiconductor die.
 27. The computer system of claim 26, wherein multiplespatially adjacent electrical-to-optical transducers in the plurality ofelectrical-to-optical transducers transmit the same signal; and whereinelectronic steering circuits in the first semiconductor die direct datato the multiple spatially adjacent electrical-to-optical transducers tocorrect mechanical misalignment in X, Y and Θ coordinates.
 28. Thecomputer system of claim 26, wherein multiple spatially adjacentoptical-to-electrical transducers in the plurality ofoptical-to-electrical transducers receive the same signal; and whereinelectronic steering circuits in the second semiconductor die direct datafrom the multiple spatially adjacent optical-to-electrical transducersto correct mechanical misalignment in X, Y and Θ coordinates.
 29. Thecomputer system of claim 21, wherein the electrical-to-opticaltransducer includes one of: a Zener diode; a light emitting diode (LED);a vertical cavity surface emitting laser (VCSEL); and an avalanchebreakdown P-N diode.
 30. The computer system of claim 21, wherein theoptical-to-optical transducer includes one of: a P-N-diodephoto-detector; and a P-I-N-diode photo-detector.